BRUSSELS — European leaders gathered Friday to discuss a stunning about-face in the orientation of the United States, Europe’s bedrock ally, amid fears that President Trump is seeking a breakup of the European Union.

The summit, the first since Trump’s inauguration, led to no concrete action plan to combat the U.S. president’s praise for Britain’s exit from the European Union, skepticism of old alliances and embrace of the Kremlin. But leaders appeared fearful that a bloc that has withstood a succession of crises in recent years would not be able to hold up against transatlantic blasts from the White House.

European countries sometimes complained that President Barack Obama neglected them in favor of a focus on Asia. But they never believed that he was actively campaigning for the destruction of a political and economic union that has enjoyed bipartisan U.S. support since it originated after World War II.

The leaders of the European Union’s 28 nations convened in Valletta, the ancient fortress capital of Malta, to discuss how to slow migration and recommit themselves to their union after Britain’s decision to file for divorce. But the unpredictable Trump shoved his way onto the agenda after a series of broadsides against the E.U.

“Who really knows what the U.S. president wants,” said French President François Hollande, who took a sharp tone against a leader with whom he spoke by telephone last week. Hollande said that Europe needed to present a united front to Washington and that no European nation should be tempted into a bilateral dalliance with the White House.

In the call with Trump, Hollande hit back at the U.S. president’s travel ban targeting majority-Muslim countries, a European diplomat briefed on the conversation said.

Trump’s victory has energized a host of right-wing parties in Europe that are skeptical — or openly hostile — to the powers in Brussels. The U.S. leader called the NATO defense alliance “obsolete” and has predicted the breakup of the E.U.

European leaders are now watching the Trump administration with increasing alarm, fearful that the United States could be the biggest threat yet to a bloc already mired in discord.

The worries have spiked to the point where European Council President Donald Tusk listed the United States alongside terrorism, Russia and China as threats to the European Union this week.

“Transatlantic cooperation remains an absolute priority because it has until now been a key pillar of the free world,” Tusk told reporters during the meeting.

But many leaders are growing worried that the pillar could be crumbling.

“We do not believe in walls and in bans,” said E.U. foreign policy chief Federica Mogherini, who added that Europe would search for “pragmatic” ways to cooperate with the United States but that it would stick to its own principles.

German Chancellor Angela Merkel, Europe’s most powerful leader and the one who has been the most resolute in pushing back against Trump, said that her continent would simply need to stand on its own during his time in office.

“Europe has its destiny in its own hands," she said.

Europe’s fears have been sharpened by an American anti-E.U. campaigner who has represented himself in interviews as Trump’s likely pick as ambassador to the European Union. Ted Malloch, a businessman, has said he was interviewed by Trump for the post.

“I had in a previous career a diplomatic post where I helped bring down the Soviet Union. So maybe there’s another union that needs a little taming,” Malloch told the BBC.

The concerns about Malloch culminated in an unprecedented pre-rejection of his candidacy by top parties in the European Parliament, which said that the E.U. should declare him persona non grata if he is nominated to the post.

The parliament has no formal role in accepting or rejecting ambassadors, but Trump’s nominee would have to be accepted unanimously by the European Union’s 28 nations along with the European Commission. A senior E.U. official said that leaders were formally studying the process by which an ambassador could be rejected.

“We would have expected this from North Korea,” not the United States, the official said, speaking on the condition of anonymity to describe sensitive internal discussions. The moves were extremely unusual given that no formal nomination has been made.

European leaders’ fears are especially high right now because the Netherlands, France, Germany and possibly Italy all face elections this year with surging populist candidates. Trump allies have built bridges to anti-elite campaigners who want to break up the European Union, and his blistering style may also be making inroads with European voters.

The far-right turmoil is “not only a threat which is caused by Mr. Trump,” Austrian Chancellor Christian Kern said. “It’s something internally in Europe.”

Some mainstream leaders could be very vulnerable, analysts say.

“If the tectonic plates are shifting under their feet, then that will shake them,” said Heather Grabbe, the head of the Brussels office of the Open Society Foundation.

The European Union’s fears come even as NATO’s worries have eased somewhat in recent days, after the confirmation of Defense Secretary Jim Mattis, a retired general who served for a time inside the defense alliance’s command structure. NATO officials increasingly believe that Trump is simply pushing for reforms, not the alliance’s breakup, two NATO diplomats said.

German leaders have been infuriated by criticism from one of Trump’s top economic advisers, Peter Navarro, who said this week that Germany was exploiting a “grossly undervalued” euro to take advantage of its trading partners. Merkel quickly hit back that Germany has no control over European monetary policy, which lies in the hands of the independent European Central Bank.

The harsh words from Washington have puzzled conventional foreign policy analysts on both sides of the aisle.

“If there were some strategy behind this, even if it weren’t shared widely, that would be one thing, but it’s not at all clear what the strategy is,” said Anthony Gardner, Obama’s final ambassador to the European Union. “There’s no conceivable hypothesis under which encouraging the fragmentation of Europe somehow serves our interests.”